In the field of friction materials, it is generally known to use porous materials to manufacture friction members, such as using porous preforms for manufacturing friction brake disks. The manufacture of such friction members generally begins with the construction of a porous preform. For example, in many friction brake applications, annular preforms are used.
The porous preforms (annular or otherwise) can be constructed using several different known methods (which are not germane to the present invention). In any event, it is desirable to further densify the resulting porous preform (especially, but not necessarily only, with a carbonaceous material) so as to obtain desired friction and mechanical properties.
Chemical vapor infiltration (“CVI”) is a widely used conventional technique in this regard for obtaining carbon/carbon composite materials. CVI uses a hydrocarbon-containing gas to infiltrate a porous preform. The CVI gas is then cracked under high temperatures so as to leave a carbon coating on the fiber structure of the preform.
Conventional CVI typically requires several hundred hours of processing in order to obtain a carbon/carbon (“C/C”) structure having a desired density and mechanical properties. By way of example, a typical, conventional CVI process includes a first infiltration cycle performed, for example, over approximately 300-500 hours or more.
However, conventional CVI frequently causes rapid blockage of the surface porosity of the preform before interior portions of the preform are adequately densified. The hydrocarbon-containing gas therefore can no longer diffuse into interior undensified parts of the preform. In order to “reopen” the surface porosity to permit further densification, an intermediate machining step becomes necessary. In general, this intermediate machining (using a known method, such as milling) removes surface layers of the preform having carbon-blocked pores to expose open pores of the preform so that the hydrocarbon gas can again infiltrate the preform structure. Taking into account that several hundred preforms are densified in a typical densification processed, the intermediate machining of individual preforms can add as much as 48 hours to the overall conventional CVI densification process.
Once the intermediate machining of the partially densified articles is completed, a second CVI process is performed to make use of the reopened surface porosity of the preforms. This second CVI process step can last, for example, another 300-500 hours or more. This generally completes the conventional densification process using CVI.
Another approach to densifying porous preforms generally uses a liquid instead of gaseous hydrocarbon precursor. This method of densification is sometimes referred to in the art as “film boiling” or “rapid densification.”
The use of liquid precursors for densification is discussed in, for example, U.S. Pat. Nos. 4,472,454, 5,389,152, 5,397,595, 5,733,611, 5,547,717, 5,981,002, and 6,726,962. The content of each of these documents is incorporated herein by reference.
Film boiling densification generally involves immersing a porous preform in a liquid hydrocarbon so that the liquid substantially completely infiltrates the pores and interstices of the preform. Thereafter, the immersed preform is inductively heated to a temperature above the decomposition temperature of liquid hydrocarbon (typically 1000° C. or more). More particularly, the liquid hydrocarbon adjacent to the inductively heated preform structure dissociates into various gas phase species within the preform porosity. Further thermal decomposition of the gas phase species results in the formation of pyrolitic carbon on interior surfaces in the open regions of the porous material, such that the porosity of the preform is reduced.
The concept of inductive heating in this field is generally known, including as described in the aforementioned references. Film boiling densification can be performed much faster than gas-based CVI processes. For example, film boiling can be substantially completed in as few several hours, versus the above-described hundreds of hours for CVI.
The benefit from faster processing times could be further enhanced by processing multiple preforms together in a process cycle. However, conventional approaches to constructing inductive heating apparatuses for treating multiple parts are electrically complex, requiring load balancing and the like.